Polyurethane sealant compositions typically comprise at least one urethane prepolymer. Sealants useful for bonding to non-porous substrates, such as glass are described, for example, in U.S. Pat. No. 4,374,237 and U.S. Pat. No. 4,687,533, both incorporated herein by reference. U.S. Pat. No. 4,374,237 describes a polyurethane sealant containing urethane prepolymers which have been further reacted with secondary amine compounds containing two silane groups. U.S. Pat. No. 4,687,533 describes a polyurethane sealant containing urethane prepolymers, which contain silane groups, which have been prepared by reacting a polyisocyanate having at least three isocyanate groups with less than an equivalent amount of an alkoxysilane, having a terminal group containing active hydrogen atoms reactive with isocyanate groups to form an isocyanatosilane having at least two unreacted isocyanate groups. In a second step, the isocyanatosilane is mixed with additional polyisocyanate and the mixture is reacted with a polyol to form polyurethane prepolymer having terminal isocyanato groups and pendant alkoxysilane groups.
Window installation in a vehicle is typically a three-stage process. First, a clear silane primer is applied to the glass to clean and prepare the surface for bonding. Second, a primer, which is essentially a carbon black dispersion which also contains a compound having silane and/or isocyanate functionality, often referred to as “black-out primer”, is then applied over the top of the clear primer. Third, an adhesive is applied to the primed glass which is then installed into the structure. For vehicles and some buildings, these materials are applied to a frit of a ceramic enamel or an organic coating located about the periphery of the window. The frit is designed to protect the adhesive from exposure to UV light and to hide the adhesive and/or trim components from view, see, U.S. Pat. No. 5,370,905, incorporated herein by reference. Further, when such compositions are used to bond glass substrates to painted substrates, such as for instance, window installation in vehicle manufacturing, the lap shear strength of the bonded substrate may be less than desirable for safety or structural purposes. Consequently, a separate paint primer comprising a solution of one or more silanes is often applied to a painted substrate prior to the application of the composition in most vehicle assembly operations for bonding the windshield and the rear window. The use of primers in assembly operations is undesirable in that it introduces extra steps, additional cost, the risk of marring the painted surface if dripped on an undesired location and exposes the assembly line operators to additional chemicals.
In order for a primer and adhesive system to be commercially viable, that system must provide a durable bond. “Durable bond” means that the adhesive holds the window into the structure for a period of years. As the structure to which window glass or plastic coated with an abrasion resistant coating is traditionally bonded lasts for a significant number of years, it is expected that the bond holding the glass or coated plastic into a structure also last a significant number of years.
It would be desirable to provide a composition which, when bonded to a painted substrate and glass or coated plastic and cured, provides a bonded structure with a higher lap shear strength, particularly when used in the absence of a paint primer and/or a primer on the glass or coated plastic. What is further needed is a system which allows for durable adhesion of the composition to the substrate surfaces.